[SI-LIST] Re: spice math question
- From: "John Ellis" <jellis@xxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Wed, 30 Oct 2002 11:28:56 -0500
Hi Andy,
Iw as using primarily modal models which are a combination of tlines and
dependendent voltage and current sources.
However...
I just did some experiments with pure 50 Ohm t-lines.
For 40 inches at td=158p/in. td should equal 6.32ns.
When I simulate a single 40 inch tline I get 5.89 ns 50% to 50% delay with a
100ps edge(0 to 100%) input.
10 sections of 4 inches each yields td=6.16ns
20 sections of 2 inches each yields td=6.3 ns.
I'd appreciate a reality check on this. I terminated the far end of the
tline in 50 Ohms to ground.
John
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Ingraham, Andrew
Sent: Wednesday, October 30, 2002 10:42 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: spice math question
Which kind of transmission line model?
If you are using the basic T-element lossless transmission lines, I
think SPICE *ought* to be accurate. I am not aware of a rounding error.
SPICE effectively implements T-element transmission lines using two raw
delays (one for each direction), a couple of controlled sources, and
resistors.
On the other hand, (1) I have been told that Hspice had problems with
T-element transmission lines at one time, and (2) if you measure
pin-to-pin, or by measuring the insertion delay due to adding a t-line,
you might not get the exact delay you ask for, depending on the
terminations on both ends. This happens in reality too.
Cascading shorter T-element t-lines *should* produce the same result.
Interesting that it doesn't.
Andy
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